As a M4/3 user, I asked a similar question on Micro 4/3 forum, and some replies were, quite understandably, just a little bit biased

So I would like to rephrase the question a bit:

- If you are on the market for a new camera,

- and you are not yet “married” to a particular lens system,

- and you are considering Olympus E-M1, what other camera would you buy if Olympus did not come up with the E-M1? In other words, what cameras does E-M1 compete with?

1. The best deal in similar cameras right now is Pentax k-30 for $470 body only (about 3 times cheaper than that E-M1). All in all it is a LITTLE BIT worse than E-M1 - 6 fps vs 6.5, 650g vs 497g (but put the included flash on E-M1 and difference becomes smaller, put comparable flash and who knows what is actually lighter), no wifi, a bit fewer controls (although 2 control wheels so it is no Rebel etc), smaller viewfinder (pentaprism though, not the cheap pentamirrors), 3-axis IBIS vs 5-axis, but the difference matters mostly in macro, plastic over steel vs mag alloy, non-tilting and non-touch screen, 1/6000 maximum shutter speed vs 1/8000. And comparable lenses are bigger on k-30. And viewfinder is OVF, so not stabilized by the IBIS, no blinkies/preview/magnification in VF etc.

But both are weather-sealed, about the same size (until you put a flash on E-M1), k-30 has flash built-in which is more powerful that the small external flash included with E-M1, and k-30 has a little better image quality at the same aperture and shutter speed.

Also, k-30 has a kit 18-135 WR, going to 200mm equivalent, pretty handy travel lens - the only similar lens for E-M1 - 12-50 - goes only to 100mm equivalent.

2. NEX-6, goes as low as $620 new for body great EVF, but no IBIS, choice of AF lenses is poor (unless you put heavy and expensive autofocusing adapter for heavy A-mount lenses, but no stabilization then). No weather-sealing, so it is not really the same class.

3. Pentax k-5 II, almost the same as k-30, only a little heavier (due to much bigger battery for much longer shooting compared to E-M1 and even k-30), LCD on top and actually slightly faster sequential shooting (7 fps vs 6.5 with AF).

4. Canon 70D, Nikon D7100, $200-300 off the price of E-M1 for better (I suspect much better) tracking AF and slightly faster sequential shooting (in case of D7100, in crop giving 2x crop factor similar to m43). But bigger/heavier, smaller viewfinders, no IBIS and no DX/EF-S lenses which are actually weather-sealed AFAIK. Also better selection of lenses, including really cheap 3rd party lenses (Sigma 18-35/1.8 alone!).

5. If you couldn't care less about tracking and sequential shooting as many have claimed in my thread, but more about landscape and low light etc, then Canon 6D (from $1500) with 24-105/4 L IS (from $660) is actually cheaper than E-M1+12-40/2.8, and IQ especially at high ISO is significantly better, viewfinder is about the same size, body+lens is weather-sealed, the gimmick of wifi is also there, and of course huge selection of lenses starting from $90 50/1.8 (not replaced even by $500 Pana 25/1.4). Body is only as heavy as K-5 II, but lenses usually bigger (just select a stop slower, you will still be a stop ahead).